Plasterboard

ABSTRACT

Plasterboard is surfaced with a lining paper which is printed over the whole of one face of the board using an ink containing binder and particulate solid material, applied with a density of print such that under optically examination at least 80% of the surface of the lining paper is covered by the applied print. The particulate material has a d50 denotes a number length mean particle size such that 50% of particles have volume smaller than a sphere of diameter d50. The colour of the printed lining paper can be matched to the colour of a jointing compound used as a filler between the edges of adjacent boards. The material of d50 mean particle size in the range from 1 to 10 micrometers gives the lining paper a surface texture resembling that of jointing compounds and so reduces or removes contrasts, notably in absorbency, reflectance and gloss, between the surface of the boards and the surface of the jointing compound between boards.

FIELD OF THE INVENTION

This invention relates to the manufacture of plasterboard (also known inUSA as gypsum wallboard). Plasterboard is well known for use in formingceilings and interior wall surfaces in buildings. Generally the frontface and side edge surface of the plasterboard have been provided by amulti-ply paper referred to as plasterboard liner, while the reverseface is surfaced with a second liner paper. Both of these liners aremulti-ply papers.

BACKGROUND OF THE INVENTION

One method for utilising plasterboard has been to fasten theplasterboard into position and then cover its exposed face and thejoints between plasterboard sheets with a thin “skim coat” of wetplaster. After this has set, it requires decoration with paint orwallpaper.

Another method for utilising plasterboard has been widely used anddispenses with the “skim coat”. The plasterboard is manufactured using aplasterboard liner paper which has a light colour. This is used to formthe front surface and the side edge surfaces. The side edges of theboards are shaped to form a recess at their abutting edges. After theboards are fastened into position, their abutting edges are joined withtape and a jointing compound which is worked into the recess on top ofthe joining tape to create a continuous flat wall surface, with jointsvisible at regular intervals. The wall is then decorated, either withwallpaper or with multiple coats of paint, which serves among otherthings to conceal the joints. If paint is used, the normal requirementis a first “mist coat” or primer, followed by two full further coats.This requires painters to come to the wall three times.

There have been a number of proposals for the manufacture ofplasterboard with a decorative finish applied during manufacture, priorto fixing the boards in place. Examples of such proposals include U.S.Pat. Nos. 3,507,684, 3,694,298 and 3,984,596.

U.S. Pat. No. 4,579,610 proposed a process in which the surface of aplasterboard liner paper is gravure printed with a clear or tinted latexbase coat in a pattern of dots, so that the applied material does notseal the paper. The base paper is then overprinted with a decorativedesign. This decorative design is not described in detail although it issaid that a pattern of blotches is preferred.

Of course, if the surface of the plasterboard is pre-decorated in someway, there can still be a contrast between the boards and a jointingcompound which is applied at the joints between boards.

WO 99/57371 discloses lining paper which is given a coating before beingmade into plasterboard. The coated surface is said to be suitable toreceive flexographic printing and there is a suggestion to print adecorative pattern of speckles. The plasterboards may be used as ceilingtiles. Joints between them are not mentioned.

EP-A-521804 (Lafarge) has described plasterboard manufactured using aliner paper with a coating applied to the liner paper before the boardsare manufactured. The document recognises that it is desirable to avoidcontrast between the colour of the board and the colour of material usedto form joints between the boards.

EP-A-1076137 (BPB plc) discloses plasterboard lining paper pre-decoratedby printing over its whole surface, with a layer of applied print whichappears continuous when inspected by eye. The colour of the print may bematched to the colour of jointing compound. Jointing compounds have beendescribed in a number of documents.

GB-A-1265804 (BPB Industries) discloses a composition containing groundlimestone, talc, mica and vinyl copolymer binder. It also teachesaddition of small percentages of bentonite clay and cellulose ether.

GB-A-2048235 (BPB Industries) discloses a jointing compound containinggypsum plaster of particle size less than 150 μm, inert mineral fillerof particle size 100 μm and polymeric binder which may be polyvinylalcohol or starch. The mineral filler may be ground limestone, and theexemplified composition includes a small percentage of bentonite.

Both these documents mention that pigment may be included in jointingcompounds. GB-A-2048235 mentions that the added colour may serve tomatch the colour of the facing paper of plasterboard.

Kanuf Bauprodukte GmbH sell moisture resistant grades of plasterboardwith a distinctive green surface, and an accompanying jointing compoundwhich is also coloured green to match the board.

WO 97/02395 (Lafarge) discloses a jointing compound intended to matchthe surface of pre-decorated plasterboard, made using a lining paperwith a coated surface, according to EP-A-521804 (Lafarge). This WO97/02395 teaches that the jointing compound should contain a highproportion of filler which has a particle size of 5 to 35 micrometers.The proprietors of WO 97/02395, Lafarge, sell pre-decorated boards and ajointing compound of matching colour.

Thus it is known to be desirable to match the appearance of thepre-decorated board surface and the jointing compound used with theboards.

We have now observed that when boards are made with pre-decorated liningpaper as taught in EP-A-1076137 and joined with a conventional jointingcompound, with a good match between the colour of the boards and thejointing compound, there may still be an observable contrast between thesurface of the boards and the exposed surface of the jointing compound.It can be described as a visible difference in the texture of the twosurfaces.

SUMMARY OF THE INVENTION

Surprisingly, we have found that this problem can be greatly amelioratedby using a printing ink incorporating particulate material of selectedparticle size. According to a first aspect of this invention there isprovided:

-   -   pre-decorated plasterboard surfaced with a lining paper which is        printed over the whole of one face of the board with an ink        containing binder and particulate solid material having a d₅₀        mean particle size in a range from 1 to 10 micrometers, with        density of print which is sufficient that under optical        examination at least 80% of the surface of the lining paper is        covered by the applied print and    -   where d₅₀ denotes a number length mean particle size such that        50% of particles have volume smaller than a sphere of diameter        d₅₀ and 50% of particles have volume larger than a sphere of        diameter d₅₀.

We have found that the use of such a particulate solid with d₅₀ greaterthan 1 micrometer gives a surface texture which is different from thesurface texture when the particulate solid in the printing ink is onlyopacifying pigment of very small particle size. A useful result is areduction in contrast between the plasterboard and a jointing compoundhaving a colouration matching that of the plasterboard.

It is possible, within the scope of this invention, that this printingink contains the contrast-reducing particulate solid material as theonly particulate material present in the ink. In this event the inkmight be somewhat translucent so that it does not hide the colourationof the underlying surface. Such an ink could be used over the top of anunderprint containing opaque pigment which did hide the colouration ofthe underlying paper.

In a preferred arrangement however, printing is carried out using aprinting ink which contains this contrast-reducing particulate solid ofthe particle size specified above and also contains other particulatematerial which is able to render the ink opaque. Thus in a preferredarrangement, printing is carried out using with an opaque ink containingparticulate solids and binder, where the particulate solids include both

-   (i) particulate material having a d₅₀ mean particle size in a range    from 1 to 10 micrometers,-   (ii) opacifying pigment having a d₅₀ mean particle size not greater    than 1.5 micrometers,    where, as before the coverage under optical examination is at least    80% of the surface of the lining paper and d₅₀ for each particulate    material denotes a number length mean particle size for that    material such that 50% of its particles have volume smaller than a    sphere of diameter d₅₀ and 50% of its particles have volume larger    than a sphere of diameter d₅₀.

BRIEF DESCRIPTION OF THE DRAWINGS

The sole drawing, which will be referred to in Example 5 below, is across section at a joint between two adjacent plasterboards.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The opacifying pigment may be a conventional particulate pigment givingopacity to the printing ink and which has very small particle size.

The first-mentioned particulate material, intended to reduce contrast insurface texture, preferably has a d₅₀ mean particle size of at least 1.5or preferably at least 1.8 micrometers. Its d₅₀ mean particle size maywell be no more than 5 micrometers, preferably no more than 4 or even 3micrometers. A particularly preferred range of d₅₀ mean particle sizefor this first particulate material is from 1.8 to 3 micrometers. Thisfirst particulate material modifies the surface texture of the printedlining paper and hence of the plasterboard made from the lining paper.As a result there is a reduction of the contrast between theplasterboard and jointing compound having a colouration matching that ofthe plasterboard. Surprisingly this improvement is observed even thoughthe particulate material incorporated in the jointing compound may havea d₅₀ mean particle size considerably greater than the d₅₀ mean particlesize of this particulate material incorporated into the printing ink.

This beneficial effect is observed even when the opaque ink on thelining paper is over-printed with a translucent or transparent secondink usually referred to as a lacquer. As taught in our EP-A-1076137 itis preferred to use this step of applying a translucent or transparentsecond ink over the first ink. It has the benefit of making thepre-decorated surface more robust and more resistant to marks by toolswhich come into contact with the surface.

The first-mentioned particulate material incorporated in the printingink preferably has a particle size distribution such that it has a d₉₀particle size which is not greater than 20 micrometers where d₉₀ denotesparticle size such that 90% of the particles have volume smaller than asphere of the diameter d₉₀. It is preferred that the first-mentionedparticulate material has a d₅₀ particle size which is less than 10micrometers, for example not more than 4 micrometers, together with ad₉₀ particle size which does not exceed 10 micrometers. The opacifyingpigment in the printing ink preferably has particle size distributionsuch that it has a d₉₀ particle size not greater than 2 micrometers.

Generally, the ratio by weight of the first particulate material toopacifying pigment in the printing ink will lie in a range from 3:1 to1:3, better 2:1 to 1:1. An opaque printing ink preferably include from25 to 75%, better 25 to 50% of the first particulate material having d₅₀mean particle size in the range from 1 to 10 micrometers and from 15 to50%, better 15 to 30% by weight of the opacifying pigment having d₅₀mean particle size not greater than 1.5 micrometers.

Plasterboard in accordance with this invention may be used in a varietyof ways, analogous to the diverse utilisations of commercialplasterboard. Thus, sheets in accordance with this invention can be usedas ceiling tiles, or used to fabricate walls or ceilings with no jointor a contrasting joint between adjacent sheets. However, the inventionis particularly useful when joints between adjacent sheets are finishedwith a composition intended to provide a continuous surface from onesheet to the next, with the joints becoming indistinguishable from theplasterboard sheets.

In a second aspect this invention provides a combination of materialsfor wall fabrication comprising sheets of plasterboard as defined aboveand a jointing compound for surfacing joints at adjacent edges of theplasterboard sheets where the colouration of the printed face of theplasterboard and the colouration of the jointing compound are arrangedto match. Both the plasterboard and the jointing compound may be whitein colour: indeed white or very pale tints are preferred.

A joint between plasterboard sheets is often formed using more than onelayer of mobile jointing compound which sets after application. Thelayers may be made using different jointing compounds. The jointingcompound referred to in the preceding paragraph is used for the finallayer so that it forms an exposed surface of the finished joint. It iswithin the scope of this invention to use the same jointing compound ora different jointing compound to form a first layer at the joint, andsubsequently conceal that first layer by application of a jointingcompound which, as specified above, is arranged to match the colourationof the surface of the plasterboard.

The numerical measurement of colour may be in terms of the CIELAB systemand it is preferred that the colouration of the plasterboard and thecolouration of the jointing compound after setting are matched such thattheir respective values of L, a and b in the CIELAB colour space satisfythe relationshipΔE=[(L ₁ −L ₂)²+(a ₁ −a ₂)²+(b ₁ −b ₂)²]^(1/2)<2where L₁, a₁ and b₁ denote the values of L, a and b of the plasterboardand L₂, a₂ and b₂ denote the values of L, a and b of the jointingcompound after the jointing compound has set. More preferably the valuesof L₁ and L₂ differ by no more than 1.3 units while the values of a₁ anda₂ differ by no more than 0.5 or 0.7 units and the values of b₁ and b₂also differ from each other by no more than 0.5 or 0.7 units.

If the boards and jointing compound are white in colour, their match incolour may be expressed by a whiteness measurement such as the WI_(CIE)Whiteness Index which is a Colorimetric method to estimate the degree ofwhiteness, recommended by the Commission International d'Eclairage in1982.

The parameters L,a,b and WI_(CIE) are related by formulae to valueswhich are measured. An ability to calculate these parameters is normallyprovided as a function of the measuring instrument.

Preferably WI_(CIE) of the plasterboard and WI_(CIE) of the jointingcompound differ by no more than 3.5 units.

Plasterboard sheets may be of conventional size. Thus, boards for use inmaking walls and ceilings may have length exceeding 2 meters and widthexceeding 70 cm, often exceeding 1 meter. Smaller boards may be utilisedas ceiling tiles, supported by a grid structure which is itselfsuspended from above.

In a further aspect this invention provides a method of fabricating aninternal wall by fixing a plurality of sheets of plasterboard as definedabove edge to edge so as to expose the printed lining paper and applyingjointing compound as defined above at joints between adjacent edges ofplasterboard sheets such that the jointing compound provides part of theexposed wall surface at the joints.

It is envisaged that jointing compound will also be used to make goodany places at which the pre-decorated surface of the plasterboard hasbeen damaged when the plasterboard was fixed in position, for example tocover the heads of any nails driven through the plasterboard.

Materials for use when putting this invention into practice will now bediscussed in greater detail.

Printing ink generally consists of solid material of small particle sizedispersed in a liquid. Organic polymers to act as binder are alsodissolved or dispersed in the liquid.

Although the use of an organic solvent as the liquid portion of the inkis possible, we prefer to use water-based inks.

The organic polymer or polymers which function as binder may be providedby a supplier of printing materials in the form of an aqueous solutionof the polymer(s) or as a dispersion of the polymer(s) in an aqueoussolution. Such a solution or dispersion may be marketed by its supplieras suitable for use as a clear or translucent lacquer.

Alternatively, such a solution or dispersion may be marketed as suitablefor mixing with pigment in order to formulate an opaque printing ink.

Particulate materials which give opacity to a printing ink generallyhave a refractive index greater than 1.6. Typically they arewater-insoluble inorganic solids. Titanium dioxide is the pre-eminentopaque white pigment. It is supplied as a powder of very small particlesize, and it may be supplied as concentrated aqueous slurry.

Other materials used as pigments with colours other than white may havelarger particle size and may be organic or inorganic compounds.

The use of white particulate materials of refractive index below 1.6 andparticle size larger than that of titanium dioxide is already known inother areas of printing technology. However, such materials do notprovide much opacity and are merely used as “fillers” or “extenders” todilute the more expensive titanium dioxide.

Printing inks for use in the present invention may be made byconventional procedures for mixing solid and liquid constituents ofinks.

It is normal practice for the manufacture of ink to be carried out by aspecialist ink manufacturer who may well be working in closeco-operation with a company having facility to manufacture and printpaper. The formulation of ink to provide a required colour is part ofthe normal skill of the ink manufacturer.

Lining Paper

The lining paper onto which the printed colour is applied will normallyconsist of several layers joined together during the papermakingprocess. The paper may be made in conventional manner using the normalmachinery employed for making multi-ply paper and board by a wet-laidprocess. The fibres used in the manufacture of the plies of the liningpaper may be new or recycled or a mixture of the two.

For the top ply it may be desirable to use fibres which have beenbleached or incompletely bleached (so-called semi-bleached fibres) sothat the top ply is a light colour before print is applied to it.Alternatively, it may be preferred to use a density of printed colourwhich is able to mask a darker colour of the liner paper. It may bepreferred that the fibres of the top ply are predominantly chemicalfibres and therefore lignin-free so as to avoid yellowing. However, ifan opaque print is applied, the invention may possibly be put intoeffect using a top ply formed with a predominance of semi-chemicaland/or mechanical fibres which retain some or all of the natural lignincontent of the original timber. Use of such fibres relies on the printedlayer to stabilise the colouration of the paper, both by reason of itsown colour and by shielding the top ply from sunlight thereby preventingor retarding yellowing of the lignin-containing fibres in the top ply.The top ply could be made from pulp—i.e. new fibres. However, for thesake of economy it will normally be made from recycled fibres,supplemented if necessary with new fibres.

Waste paper to provide the recycled fibres will generally be selected togive a light off-white, shade. Suitable sources of mechanical fibreswhich retain the natural lignin include unprinted newspaper, lightlyprinted paper and so-called woody trimmings—i.e. off cuts from papermade using mechanical pulp.

Beneath the top ply there may, if desired, be a second ply also having ahigh proportion of bleached fibres and possibly of similar compositionto the top ply. The remaining plies form the interior and the reverseface of the lining paper. It is normal for these to contain a highproportion of unbleached fibres which may be chemical fibres, becausetheir strength is greater than that of mechanical fibres. A suitablesource of recycled fibre for these interior plies is recycled corrugatedboard from boxes and cartons.

Unsorted mixed waste paper B which is the residue after removing highervalue light-coloured material B will generally contain a substantialproportion of unbleached chemical fibres from boxes and cartons. It maywell be used as the source of recycled fibre for these interior plies.

A plasterboard lining paper conventionally has a weight in a range from150 to 300 grams per square meter (gsm) and usually lies in a narrowerrange from 180 to 250 gsm. Paper within this range is suitable for usein this invention.

The porosity of plasterboard lining paper can be determined by theGurley test of porosity laid down in the British Standard BS 6538: Part3: 1987 entitled “Air Permeance of Paper and Board”. This corresponds toISO 5635/5—1986 and French standard NFQ03-061.

This test measures porosity as the time for a known volume (100 ml) ofair to pass through a sample of predetermined area (6.4 cm²). The resultis therefore expressed in seconds/100 ml or simply in seconds becausethe 100 ml air volume is standard.

Printing Process

Printing onto the paper liner is most easily done before the liner isused in plasterboard manufacture. It is preferably done using a printingprocess which applies a continuous layer of ink to the substrate whichis being printed. A process which is particularly envisaged isflexographic printing. The flexographic process is widely used forprinting onto fibreboard for boxes, paper for paper sacks, and otherpackaging materials. In that application the print is applied to thepaper or board from, for example, a “stereo” or a moulded printingroller which is formed using a polymeric material and is a mirror imageof the design or wording which is being printed.

Continuous printing along a web (as contrasting with printing arepeating image) is a technique which is already available in theprinting industry. For printing over substantially the whole width of aweb, a normal flexographic printing machine can be used, but the rollerwhich would normally carry a stereo is instead provided with acontinuous surface of the polymer material or a printing roller with asmooth surface can be used.

Where the continuous surface is provided by a flexible sheet ofpolymeric material wrapped around a roller, edges of the sheet whichextend along a circumference of the roller are butted against eachother.

Suppliers of sleeves, stereos and moulded rollers for printing includeRegal Rubber, Miller Graphics, Design and Stereo Services and Strachan &Henshaw machinery.

Printing could be carried out by another printing process, so long asthe result is to apply print to over 80% of the surface to which printis applied. In order to control the colour of the printed liner paper itis desirable that coverage is even higher, such as at least 90% or atleast 95% of the area.

Printing by a gravure process at more than one printing station would bea way to cover a high proportion of the total area: gravure dots appliedat one printing station could largely fill the gaps between dots appliedat another, although overlap of dots would also occur.

Whatever technique is employed it is likely that the weight of solidsapplied, including binder resin, will lie in a range from 7 to 30gram/m².

The lining paper web which receives the print will generally have awidth slightly greater than the width of the plasterboards which will bemade, so that it can cover one face and two edges of the boards and alsowrap around onto the margins of the reverse face where it will beoverlapped by the second web.

Preferably the print is applied to the entire width of one surface of aweb of lining paper and in consequence the resulting plasterboard hasthe printed colour completely covering the face of the board, twoopposite edges of the board and the margins of the reverse face.

Alternatively, the edge portions of the web which form the edges of theboard and wrap around onto the reverse face may be left unprinted. Forthe sake of good appearance it is preferred that the printed colourextends from the exposed face onto at least part of each edge.

Even with flexographic printing we prefer to print at more than oneprinting station in order to obtain a combination of properties.

In a preferred arrangement, the first layer of print to be applied is anopaque ink containing both particulate material with d50 in the range 1to 10 micrometers, as required by this invention and also containingopacifying pigment. Such an ink may well contain more than 40% by weightof particulate solids, as well as binder, water and minor ingredients.The particulate material is likely to constitute more than 50% of thetotal solids in the ink.

A top layer of print may then contain a much lower proportion ofparticulate solid or even none at all, e.g. 0 to 25% of the solidscontent, with a higher proportion of binder resin. Such a layer canserve as a lacquer, giving an improved resistance to marking duringhandling and to abrasion after wetting. The latter is valuable forgiving a “wipe-clean” or washable surface.

Plasterboard Manufacture

As mentioned in EP-A-1076137, it has been found that the application ofa continuous printed colour can lead to a large reduction in porosity.The Gurley porosity after printing may lie in a range from 200 to 400seconds. Surprisingly, however, the manufacture of plasterboard usingthe printed paper can be carried out by entirely conventional steps. Ina typical continuous production process these steps will be:

-   -   unreeling of lining paper onto a production line with the        eventual outer face (in the case of the present invention the        printed surface) at the underside;    -   delivery of wet plaster onto the web of lining paper;    -   shaping of the edges of the web to form edges of the eventual        plasterboards;    -   application of a second web of lining paper to form the reverse        face of the boards;    -   cutting of the resulting continuous material into individual        sheets; and    -   passage of these cut sheets through a kiln to dry the plaster.

The steps of introducing wet plaster between two webs of lining paper,shaping edge portions, cutting into lengths and drying the boards willgenerally all be carried out in conventional manner on automatedmachinery.

The lining paper web which received the print will generally have awidth slightly greater than the width of the plasterboards which will bemade, so that it can cover one face and two edges of the boards and alsowrap around onto the margins of the reverse face where it will beoverlapped by the second web. If the print is applied to the entirewidth of one surface of a web of lining paper, the resultingplasterboard has the printed colour completely covering the face of theboard, two opposite edges of the board and the margins of the reverseface.

Alternatively, the edge portions of the web which form the edges of theboard and wrap around onto the reverse face may be left unprinted. Forthe sake of good appearance it is preferred that the printed colourextends from the exposed face onto at least part of each edge.

Jointing Compounds

Jointing compounds for use in this invention may be made in accordancewith existing technology for such compounds.

A suitable composition contains a majority of an insoluble particulatefiller. Calcium carbonate may be a convenient choice. Otherpossibilities are hydrated or anhydrous calcium sulphate and alsodolomite.

A suitable size range is a d₅₀ value from 10 or 15 micrometers up to 100or 200 micrometers.

In order to give a smoother surface finish, a small percentage ofsmaller particle size material may be incorporated. A suitable range ofparticle size for such material is from 0.1 or 0.5 micrometers up to 10or 15 micrometers. The percentage which may be included is likely to besmall, no greater than 5% of the composition typically in the range upto 2% by weight of the composition. One material which is available withsuitable particle size is talc.

Mica particles may be included in a small proportion, typically no morethan 5%, possibly no more than 2% by weight of the composition. Mica hassmall particle size and also has the benefit of reducing shrinkage as acompositions sets.

Bentonite clay may be included in small amount to thicken the overallcomposition. The amount of clay is likely to be no more than 5%,possibly no more than 2% by weight of the composition.

Another class of materials which may be utilised to increase viscosityof a composition is cellulose ethers. Their amounts are likely to be nogreater than 1%, probably no more than 0.7% by weight of thecomposition, especially if bentonite clay is included.

Suitable binder polymers for incorporation in jointing compositionsinclude polyvinyl alcohol as taught in GB 2048235 A, polyvinyl acetateand mixtures of the two.

Some binder materials react chemically after the composition is applied,and cause it to set. Others rely on evaporation of moisture so that thecomposition hardens as it dries.

Minor ingredients which may be present in a jointing composition includeantifoams, antibacterial compounds, and coloured pigment.

Jointing compositions are made by mixing the constituent solid withwater. They may be supplied as powders and mixed with water on site togive a viscous paste or they may be mixed with water by themanufacturer—who can then use mixing machinery in a factory—and sold insealed containers. A typical water content is in a range from 28 to 35%of the jointing compound, approximately corresponding to water:powderratios of 0.4 to 0.55.

Test Procedures

There are a variety of methods for particle size determination, and avariety of weightings which can be incorporated when calculating meanvalues.

For measuring sizes of particulate constituents of printing inks, wehave used a Malvern Mastersizer instrument, with a Hydro 2000 sampledispersion accessory to allow observation of particles in liquidsuspension. This well-known instrument uses low angle laser lightscattering, more commonly known as laser diffraction. This techniquedetermines the volumes of individual particles, from which averageparticle size can be calculated in several ways, using computer softwarewhich accompanies the Malvern Mastersizer instrument.

The size of an individual particle is taken as the diameter of aspherical particle of the same volume, the so-called “equivalentsphere”.

The software associated with the Mastersizer instrument enablesdetermination of “number length mean size” denoted as d(0.05) or d₅₀.This is a mean value of particle size such that 50% (by number) of theparticles have a particle size smaller that this value and 50% (bynumber) of the particles have particle size larger than this mean value.

Thus, d₅₀ is a mean size such that 50% of the particles have a volumelarger than the volume of a sphere or diameter d₅₀ and 50% of theparticles have a volume smaller than the volume of a sphere of diameterd₅₀.

Colour Measurement

The CIE (L*a*b*) colour space is a specification of colour perceptionsin terms of a three dimensional space. It was one of two systemsspecified by the Commission Internationale D'Eclairage in 1976. It isalso known as the CIELAB formula. The parameter L is the brightnessco-ordinate. It is a measure of the brightness of the sample on a greyscale from white=100 to black=0.

-   a is the red/green co-ordinate with positive values indicating red    and negative values indicating green.-   b is the yellow/blue co-ordinate with positive values indicating    yellow and negative values indicating blue. The difference between    two colours can be expressed by the formula    ΔE=[ΔL ² +Δa ² +Δb ²]^(1/2)    where ΔL, Δa and Δb are the differences between the L, a and b    values of the two colours.    Colourmetric measurements were made using a Minolta CM 508i    instrument to determine values of L, a and b according to the CIELAB    system.

The same instrument was used to calculate whiteness as a value of theCIE Whiteness Index (WI_(CIE)) as defined by the CommissionInternationale d'Eclairage in 1982.

Gloss was determined using a Minolta Multi Gloss 268 instrument at anangle of incident light of 85° to the normal, which is of course a lowangle of only 5° to the surface.

Measurement was made parallel to the machine direction of the paper(denoted as MD Gloss) and perpendicular to this direction, i.e. in thecross direction (denoted as C D Gloss). Gloss of jointing compound aftersetting was measured parallel to the machine direction of the paper,which is along the length of the joint and traverse to the paper andjoint.

Porosity—Gurley

The porosity of paper was determined by the Gurley test mentioned abovewhich measures the time (quoted in seconds) for 100 ml of air to passthrough a sample area of paper with an areas of 6.4 cm².

Surface Roughness—Bendtsen

The Bendtsen test for surface roughness can be applied to paper or toplasterboard It is defined by French Standard NF Q 03-049.

Surface Water Absorbency—Cobb

The Cobb test for water absorption is primarily a test which can beapplied to paper. It is defined by ISO 535 (also French Standard NF Q03-014 and British Standard 2644). In this test a 100 cm² area ofsurface is defined by means of a ring. Water is placed within the ringunder specified conditions for a specified time of 1 minute after whichthe surplus water is removed and the water-uptake by the test sample isdetermined as a gain in weight of the sample. The result may beexpressed in g/m². It is possible to apply the Cobb test procedure toplasterboard or to an area of set jointing compound, provided this issufficiently flat. The results from this test will be affected by theabsorptive power of the board or joint material below the surface, andthe conditions of storage of samples prior to testing may affect theresults obtained.

Water Drop Test

This is another test of water absorption. It requires a smaller area ofsample than the Cobb test. In this test one drop of water, having volume0.05 cm³ is placed on the test surface and the time for completeabsorption is measured. The procedure is repeated on different areas ofthe sample and the results are averaged. Once again, this is a testwhere the result will be affected by the absorptive power of the boardor joint material below the surface. Conditions of storage of samplesprior to testing may affect the results obtained. The utilisation ofthis test and the Cobb test above therefore needs some caution. It willbe desirable to apply the test to some form of control at the same timeas any test samples.

Wet Abrasion

The wet abrasion test can be applied to paper or plasterboard. It isdefined by German Standard (DIN) 53778. It is carried out by using amechanism to move a brush across the surface under test in astandardised manner, while it is exposed to a humid atmosphere. Thenumber of passes before the surface deteriorates is recorded.

Marking

This test is an observation as to whether the metal tools used to applythe jointing compound make dark marks on the light coloured surface ofthe pre-decorated plasterboard.

EXAMPLE 1

A number of water-based flexographic printing ink compositions wereprepared using the following materials:—

-   -   Hydrobase Blanc Z10-070, an aqueous slurry containing 78% by        weight titanium dioxide white pigment, supplied by BASF Systemes        d'Impression, Clermont, France.    -   Vernis Q-40-001, an aqueous suspension of organic polymer to act        as a binder. The solids content of the suspension, believed to        be mostly organic polymer, is about 39% by weight. This        composition is also supplied by BASF Systemes d'Impression,        Clermont, France.    -   Calibrite S.L. which is calcium carbonate    -   Hydrocarb which is calcium carbonate supplied by Omya S.A.S.,        Paris    -   Talc de Luzenac 20M2,    -   DRB3 which is dolomite supplied by AGS BMP    -   Revetcarb which is a calcium carbonate supplied by BHTL—MEAC    -   DHBleu which is micronised gypsum supplied by BPB Placo, France.        The d₅₀ mean particle sizes of the particulate materials and        also their d₁₀ and d₉₀ values were determined with a Malvern        Mastersizer instrument. The meaning of d₅₀ and d₉₀ has been        given above. Analogously, d₁₀ is a size (in micrometers) such        that only 10% of the particles have volume smaller than a sphere        of diameter d₁₀.

Flexographic inks were prepared by mixing one or more of the particulatematerials with the binder suspension Q-40-001.

In a preliminary experiment, plasterboard lining paper was printed withthese inks, but not thereafter overprinted. The samples of paper wereexamined by light microscopy. They were assessed for five propertieswhich were

-   -   1. size of filler aggregate particles deposited in the printing        step    -   2. coverage of paper fibres (expressed as a percentage)    -   3. filling of the spaces between paper fibres (expressed as a        percentage    -   4. uniformity of the aggregate particles deposited in the        printing step    -   5. opacity of the printed layer, i.e. whether or not it        concealed the original colour of the lining paper.

The results are set out in the following table which also gives the inkformulations.

EVALUATION WITH BINOCULAR MICROSCOPE (×200) INK COMPOSITION Uniformityof GRANULOMETRY Q40- Added Filling deposited Size of d₁₀ d₅₀ d₉₀ Solids001 water Deposited Coverage of between aggregate aggregate SOLIDS (μm)(μm) (μm) (wt %) (wt %) (wt %)** particles Fibres fibres particlesparticles Opacity DRB 3 1 2.8 8 30 70 5 Fine Medium Good Very Good <10μm Translucent (AGS-BMP) Revetcarb 1.4 7.7 45.7 30 70 5 Medium MediumGood Medium 10-20 μm Translucent (BHTL MEAC) Calibrite 2.4 19.8 41.1 3070 5 Medium Low Medium Low 10-30 μm Translucent (OMYA) Hydrocarb + 1.14.4 30.5 20 + 20 60 5 Fine to Medium Medium Medium 10-30 μm TranslucentCalibrite Medium Talc 3.8 10.3 22.8 30 70 5 Medium Very Good Good Low 10μm Translucent (Lusenac) DH Bleu 2.6 17.9 51.7 30 70 5 Coarse MediumMedium Low 20-50 μm Translucent (BPB) Hydrocarb 1 2.6 7.5 30 70 5 FineGood Good Very Good <10 μm Translucent (OMYA) TiO₂ 0.4 0.7 1.1 88.8% of10.7 5 Very Fine Medium Medium Good <<10 μm Opaque (BASF)* hydrobase*The opaque ink containing titanium dioxide was made by mixing 88.8% ofHydrobase with 10.7% of the polymer suspensions Q40-001. The Hydrobasecontained 78% by weight titanium dioxide. **The amount of water added isgiven as a percentage of the total (100%) of the weight of particulatesolid and polymer suspension Q40-001, or Hydrobase and Q40-001.

As indicated in the upper part of the table, the best uniformity ofprinted deposit was given by Hydrocarb and DRB23 which had d₅₀ valuesbetween 2 and 3 micrometers and also had a narrow particle sizedistribution as indicated by their d₉₀ values.

RevertCarb, with larger d₅₀ and d₉₀ values was not so good. Neither wasthe 1:1 mix of Hydrocarb and Calibrite. The others, with d₅₀ valuesabove 10 micrometers and higher d₉₀ values were even poorer, especiallyas regards uniformity of the deposit on the paper.

None of these printing inks gave effective covering of the top ply ofthe lining paper. Opaque coverage was only achieved with the inkcontaining TiO₂.

When examined by scanning electron microscope it could be seen thatCalibrite, DH Bleu, the Hydrocarb and Calibrite mixture, and (to someextent) Revetcarb allowed the paper fibres to remain visible. These arethe materials which have higher d₉₀ values.

Titanium dioxide used alone gave a smoother coating than anything else.

Talc appeared to give good coverage of fibres but the printed surfacewas more uneven than the surfaces provided by Hydrocarb and DRB 3.

EXAMPLE 2

A further ink was made using Hydrocarb and also including Hydrobase toprovide TiO₂. This had the following formulation:

35.7% Hydrocarb 28.6% Hydrobase Z-10-070 35.7% Binder suspension Q40-001Water was added in an amount which was 7.1% of the total (100%) of theabove.

-   d₅₀ for the mixture of Hydrocarb and titanium dioxide was found to    be 4.0 micrometers.-   d₉₀ for the mixture was 8.2 micrometers.

This ink gave very good results:

Deposited particles: Fine Coverage of fibres: 50-75% Filling of spaces:50-75% Uniformity of deposit: Uniform Opacity: OpaqueWhen viewed by scanning electron microscope the surface resembled thesurface texture provided by the ink which contained Hydrocarb as theonly particulate material.

EXAMPLE 3

Flexographic ink was made from a combination of 85% of Hydrobase Z10-070and 15% of the organic polymer binder suspension Vernis Q-40-001 both asreferred to in Example 1. This ink was printed onto lining paper withthe amount of ink applied being either 6 or 8 grams per sq meter.

There was no over printing with transparent lacquer. The pre-printedlining paper was used to make plasterboard and sheets of theplasterboard were then joined with a commercial jointing compoundbetween adjacent edges. It was observed that the tools used to carry outthe application of jointing compound left dark marks on the surface ofthe plasterboard.

In a subsequent experiment plasterboard lining paper was printed with anink consisting of 87% Hydrobase Z10-070 and 13% Vernis Q40-001.

Some of the lining paper was then overprinted with Vernis Q-00-090, anaqueous suspension of organic polymer to act as a binder giving a mattefinish, with a solids content of 39%. This is supplied by BASF Systemesd'Impression and sold by them for use as a clear lacquer. The amountapplied by printing was 7.5 gms per sq meter so that after dryingapproximately 3 gms per sq meter of organic polymer had been applied.

The printed lining paper, both with and without the over-print oftransparent lacquer was used for making plasterboard. Sheets of theplasterboard were then joined using a commercial jointing compound. Itwas observed that tools for applying the jointing compound made marks onthe sheets which did not have the over-printing with transparent lacquerbut did not make marks on the sheets which did have over-printing withtransparent lacquer.

EXAMPLE 4

Plasterboard lining paper was printed and then made into plasterboard.The lining paper which was used had the following characteristics beforeprinting:

Weight, as measured 225.4 gm/m² Resistance to wet abrasion 11 to 14cycles Gurley porosity 190 seconds Cobb, 1 minute, on front face 17.1g/m² Cobb, 1 minute, on reverse face 20.3 g/m² L 90.16 a 0.458 b −0.094WI_(CIE) 75.16 MD Gloss 6.5 CD Gloss 4.9 Bendtsen 500 ml/min

It was first printed with an opaque ink, then overprinted with one orother of the following transparent lacquer formulations, both suppliedby BASF Systemes d'Impression.

-   -   Vernis Q-00-090, as used in Example 3, an aqueous suspension of        organic polymer to act as a binder giving a matte finish, solids        content 39%. It is sold for use as a clear lacquer.    -   Vernis Q-816512, an aqueous suspension of organic polymer to act        as a binder, giving a matte finish and also containing an        anti-slip agent, total solids content 39%. It is sold for use as        a clear lacquer.

The ink compositions used are set out in the following table which alsogives characteristics of the printed paper. The amount of opaque inkapplied was in the range 8 gm/m² to 12 gm/m², which in either case wassufficient to provide an opaque print, concealing the colour and textureof the lining paper beneath. The amount of transparent lacquer appliedwas 7.5 gm/m².

Example no 4A 4B 4C 4D 4E 4F 4G 4H Opaque Vernis Q40-001 14.8 13.0 10.846.2 46.2 35.7 35.7 35.7 ink (wt %) composition Hydrobase Z10-070 84.786.7 88.7 7.7 7.7 28.6 28.6 28.6 (wt %) Hydrocarb (wt %) 0 0 0 23.1 23.135.7 35.7 35.7 Calibrite (wt %) 0 0 0 23.1 23.1 0 0 0 Added water (% bynot not 30 8 7.1 7.1 7.1 7.1 wt of above) measured measured Solidsdeposited not not 4.52 6.41 6.46 6.65 6.65 6.65 from the opaquedetermined determined ink (g/m² ) Transp. Vernis Q00-090 100 100 100 100100 100 0 0 lacquer (wt %) Vernis Q816512 0 0 0 0 0 0 100 100 (wt %)Solids deposited 2.93 2.93 2.93 2.93 2.93 2.93 from the lacquer (g/m² )Properties Gurley Porosity 287 270 330 327 440 of printed (sec) paperWet abrasion 12-15 8-13 9-13 7-11 (cycles) Cobb 1 min at 20° C. 23 2019.4 19.1 21 front face (g/m²) Cobb 1 min at 20° C. 15 21 20.9 20.6 16rear face (g/m²) L 91.86 90.21 90.61 90.17 90.57 91.32 91.45 91.43 a−0.75 0.68 −0.64 −0.82 −0.05 −0.11 −0.17 −0.10 b 2.35 6.35 3.36 4.521.39 1.27 1.23 1.41 WI_(CIE) 61.6 37.9 61.6 54.7 70.4 72.8 73.3 72.9 MDGloss 3.4 2.9 5.1 3.9 4.2 8 7.4 7.6 CD Gloss 2.8 2.4 4.6 3.7 3.4 6.4 5.85.9 Bendtsen 900 500 1814 480 500 350

Some of these printed papers were then used to manufacture plasterboardon a conventional plasterboard production line.

Plasterboard sheets made using these pre-decorated lining papers werethen fastened in place. Joints between them were made using variouscommercially available jointing compound intended to match white boards,or else a jointing compound of the following composition (referred to asJC1).

Composition JC1 Trade Name Description % by Weight Beatite 16 Groundlimestone 67.68 Bermocoll E481Q Cellulose ether 0.52 Panther A4Bentonite clay 0.75 Mowilith DM 292F Vinyl acetate 5.52 (Hoechst)copolymer BYK 035 Antifoam 0.13 BX Fungicide 0.21 Ultramarine BluePigment 0.008 Water 25.18

The d₁₀, d₅₀ and d₉₀ values for samples of Beatite 16 and Panther A4were determined using the Malvern Mastersizer, and found to be

d₁₀ d₅₀ d₉₀ Beatite 16 1.98 μm 23.12 μm 92.16 μm Panther A4 3.58 μm20.53 μm 68.74 μm

In the above jointing compound formulation the Bermocoll cellulose ethergives water retention and helps to increase the viscosity/stability ofthe jointing compound prior to use. Mowilith DM 292F from Hoechst is avinyl acetate copolymer with acts as a binder.

Samples of board and samples of set joints between the boards weretested using some of the tests specified above. The match between boardsand jointing compound was also observed visually, both looking directlyat the joined boards and looking at an angle of 15 to 20 degrees to thesurface. Observations were made by two, sometimes three observers. Ineach instance, the observers were unanimous in their judgement. Resultswere as shown in the following table which gives properties of boards,jointing compound after setting, and match between them.

The jointing compounds are denoted by codes. P and L were available ascommercial products. Jpb1 and M10 were made for use in these experimentsand JC1 is the jointing compound of formulation given above.

Paper and plasterboard of Example no 4A 4B 4D 4E 4F 4G 4H Properties ofL 91.76 90.21 90.22 90.56 91.28 91.48 91.33 plasterboard a −0.79 0.68−0.93 −0.15 −0.18 −0.17 −0.24 b 3.95 8.14 5.82 2.89 2.72 2.78 2.96WI_(CIE) 61.61 37.95 48.79 63.36 65.97 66.25 64.98 Marking No No No NoNo No No MD Gloss 3 3.7 2.2 2.2 4.2 4.8 4.2 CD Gloss 2.5 2.8 1.9 1.9 3.23.8 3.3 Wet abrasion 7-10 4-7 9-15 9-13 9-16 4-6 (cycles) Bendtsen 9002300 1200 1100 1200 Properties of Jointing P M10 Jbp1 L L JC1 JC1jointing Compound compound L 90.4 91.25 94.17 90.16 90.16 90.64 90.64 a0.3 1.08 0.78 0.25 0.25 −0.38 −0.38 b 3.2 7.81 5.73 2.79 2.79 2.39 2.39WI_(CIE) 61.4 42.28 59.5 62.82 62.82 65.88 65.88 MD Gloss 0.8 0.7 0.70.8 0.8 0.4 0.4 CD Gloss 0.6 0.7 0.8 0.7 0.7 0.4 0.4 Comparison Δ L(pref. <1.3) 1.36 1.04 3.95 0.4 1.12 0.84 0.69 Δ a (pref. <0.5) −1.090.4 1.71 0.4 0.43 0.21 0.14 Δ b (pref. <0.7) 0.75 0.33 0.09 0.1 0.070.39 0.57 Δ WI_(CIE) 0.2 4.33 10.71 0.54 3.14 0.37 0.9 (pref. <3.5) Δ E(pref. <1.5) 1.9 1.16 4.31 0.57 1.20 0.95 0.91 Δ MD Gloss 2.4 3 1.5 1.43.4 4.4 3.8 Δ CD Gloss 1.7 2.1 1.1 1.2 2.5 3.4 2.9 Visual assessment, MC C M M M M viewing face-on M = Match C = Contrast Visual assessment atC C C C M M M 15° angle M = Match C = Contrast

The values of L, a and b for the plasterboards in the above table areall fairly similar and indicate a pale colour close to white. In most ofthe examples the colouration of the jointing compound was similar sothat the values of ΔE are less than 1.5. Example 4D was the mainexception: in this Example the jointing compound had a lighter colourthan the plaster boards, indicated by its higher values of L andWI_(CIE). This illustrates an observable mis-match of colour between theboards and the jointing compound used with them.

With the boards of Example 4A where the print applied to the liningpaper contained titanium dioxide pigment without Hydrocarb, a bettermatch of colour was achieved and very similar whiteness. When the boardsand joint were viewed face on no contrast could be seen between theboard and the joint. However, when viewed at an angle of about 15° tothe surface a contrast was observed attributable to differences intexture between the surface of the pre-decorated plasterboard and thesurface of the jointing compound.

In the case of Example 4E, the ink applied to the lining paper includedboth Hydrocarb and Calibrite in addition to the titanium dioxideopacifying pigment. A good match of colour and whiteness was achieved asshown by the low LE value of 0.57 and low ΔWI_(CIE) value of 0.54. Whenthe board was viewed face on it appeared to match the jointing compound.However, once again when the board and jointing compound were viewed atan angle of 15° to the surface, a contrast between them was observable.

In the case of Examples 4F, 4G and 4H, there was an acceptable match ofcolour and whiteness. No contrast between the boards and the jointingcompound was observable when assessed visually by viewing at an angle of15° to the surface.

These experimental results demonstrate the advantage of boards accordingto the invention: a good match between the surface of predecoratedboards and the surface of joints between them. This will be apparentwhen boards of the invention are used to make the interior walls of aroom: it is inevitable that an observer standing within the room willsee parts of its walls at an angle.

Some further properties of boards and jointing compounds wereinvestigated.

Samples of board according to Examples 4G and 4H were joined usingjointing compound JC1 and then subjected to a test in which wallpaperwas applied to the boards and joint, allowed to dry for 48 hours andthen removed. After that, the boards were allowed to dry for 72 hoursand then the same cycle of applying wallpaper, leaving it to dry andremoving it was repeated. This cycle of applying and removing wallpaperwas carried out repeatedly as many times as possible. No paint or primerwas applied to the boards before the wallpaper. Removal of the wallpaperwas carried out by exposing the wallpaper to steam until soft and thenscraping it off the walls with a metal scraper.

It was observed that with these boards embodying the invention, papercould be applied and removed five times. When paper was applied for asixth time and then removed, the boards showed slight damage and marksfrom the scraper used to remove the wallpaper.

The same test was applied to commercial boards marketed by Lafarge undertheir trade mark Pregydeco. The lining paper on these boards had a whitecoating and was believed to be in accordance with EP 521804.

Wallpaper was applied to these boards, left to dry for 48 hours and thenremoved. After this cycle of applying and removing wallpaper had beencarried out three times, the boards showed damage to such an extent thatthe boards were not in a suitable condition to be wallpapered again.

The above tests were carried out using a light-weight wallpaper and astandard wallpaper adhesive. The same results were obtained when aheavy-weight wallpaper was used and an appropriate heavier adhesive.

Boards of Example 4G above and a joint made with the above jointingcompound JC1 were exposed to sunlight for a period of 120 hours. The CIEparameters L, a and b were measured before and after this test period.The changes in value of L, a and b were too small to be visible to thehuman eye.

The Cobb test and water drop test procedures were applied to samples ofboards according to Example 4H and set joints between them made with thejointing compound JC1 above. As a comparison, the same measurements weremade on samples of conventional plasterboards and jointing compound.Each test was carried out four times. Average values are given in thetables below, together with the highest and lowest values recorded inthe four tests.

Cobb test results (g/m²) Standard Plasterboard of commercial Example 4H,plasterboard with Jointing and jointing compound JC1 compound Cobb ofAverage 18.6 15.7 Board Lowest 18.1 11.9 Highest 19.3 18.7 Cobb ofAverage 117.7 102.4 Joint Lowest 111.8 80.3 Highest 123.1 130.6

Water drop test results (minutes) Standard Plasterboard of commercialExample 4H, plasterboard with Jointing and jointing compound JC1compound Value Average 129.8 142.9 for Lowest 128.4 141.6 Board Highest130.5 144.1 Value Average 56.7 45.8 for Lowest 54.6 45.4 Joint Highest62.0 46.1

The results in the table above show that the boards of Example 4H andthe jointing compound JC1 differ considerably in their surfaceabsorption properties, analogously to conventional plasterboard andjointing compound.

Nevertheless, when samples of plasterboard of Example 4H joined withjointing compound JC1 were painted, it was possible to obtain aspecified standard of finish without prior application of a primer coat.In contrast, samples of conventional commercial plasterboard joined witha commercial jointing compound required an initial primer coat in orderto reach the same standard of finish.

It was observed that boards of Example 4G slid over one another rathereasily and as a result boards piled one upon another would slip when thepile was tilted to an angle of approximately 33° which is consideredunsafe. By contrast when the clear lacquer containing an anti-slip agentwas used in Example 4H, the slipping of one board against another wasreduced and a pile of boards could be tilted to 38 or 39° before theybegan to slip.

EXAMPLE 5

The sole drawing is a cross section through two adjacent plasterboardsaccording to Example 4H at a joint between them.

Each plasterboard sheet 10 has the predecorated lining paper on itssurface 12. The joint between the boards is filled with a first layer 14of a jointing compound in which a tape 16 is embedded. A second layer 18of jointing compound provides an exposed surface 20 flush with thesurfaces 12.

The jointing compound for layer 18 is compound JC1, which has palecolouration matching that of the surfaces 12. The layer 14 is notexposed to view. Consequently, it may be formed from jointing compoundJC1 or alternatively from some other jointing compound which does notmatch the colouration of surfaces 12.

1. A system of materials for wall fabrication comprising: (a)plasterboard, and (b) a jointing compound, wherein the plasterboard issurfaced with a lining paper which is printed over the whole of one faceof the board with an ink containing binder and particulate solidmaterial having a d₅₀ mean particle size in a range from 1 to 10micrometers, with a density of print such that under optical examinationat least 80% of the surface of the lining paper is covered by theapplied print, and wherein the jointing compound contains particulatematerial and binder, the articulate material having a d₅₀ mean articlesize in the range from 15 to 200 micrometers, where d₅₀ denotes a numberlength mean particle size such that 50% of particles have volume smallerthan a sphere of diameter d₅₀ and 50% of particles have volume largerthan a sphere of diameter d₅₀.
 2. The system according to claim 1 wherethe lining paper is printed over the whole of one face of the board withan ink containing opacifying pigment having a d₅₀ mean particle size notgreater than 1.5 micrometers and thereafter over printed over the wholeof the same face of the board with said ink.
 3. The system according toclaim 1 wherein the particulate solids in the ink include both (i) saidparticulate material having a d₅₀ mean particle size in a range from 1to 10 micrometers, and (ii) opacifying pigment material having a d₅₀mean particle size not greater than 1.5 micrometers, where d₅₀ denotes anumber length mean particle size such that 50% of particles have volumesmaller than a sphere of diameter d₅₀ and 50% of particles have volumelarger than a sphere of diameter d₅₀.
 4. The system according to claim 1wherein the lining paper is also over printed with a less opaque inkover the first said ink.
 5. The system according to claim 1 wherein theparticulate material has a d₅₀ mean particle size in a range from 1.5 to4 micrometers.
 6. The system according to claim 1 wherein theparticulate material has a d₉₀ particle size not greater than 20micrometers, where d₉₀ denotes a particle size such that 90% of theparticles have volume smaller than a sphere of diameter d₉₀.
 7. Thesystem according to claim 6 wherein the particulate material has a d₅₀mean particle size in a range from 1.5 to 4 micrometers and a d₉₀particle size not exceeding 10 micrometers.
 8. The system according toclaim 2 wherein the opacifying pigment has a d₉₀ particle size notexceeding 2 micrometers.
 9. The system according to claim 1 wherein thebinder in the ink is one or more organic polymers.
 10. The systemaccording to claim 1, wherein the colouration of the plasterboard andthe colouration of the jointing compound after setting are matched suchthat their values of L, a and b in the CIELAB colour space satisfy therelationshipΔE=[(L ₁ −L ₂)²+(a ₁ −a ₂)²+(b ₁ −b ₂)²]^(1/2)<2 where L₁, a₁ and b₁denote the values of L, a and b of the plasterboard and L₂, a₂ and b₂denote the values of L, a and b of the jointing compound after settingthereof.
 11. The system according to claim 10 whereL ₁ =L ₂±1.5a ₁ =a ₂±0.7b ₁ =b ₂±0.7.
 12. A method of fabricating an internal wall by fixing aplurality of sheets of plasterboard as defined in claim 1 edge to edgeso as to expose faces surfaced with said lining paper, applying jointingcompound as defined in claim 1 at joints between adjacent edges ofplasterboard sheets so as to provide part of the exposed wall surface atthese joints.
 13. The system according to claim 3 wherein the opacifyingpigment has a d₉₀ particle size not exceeding 2 micrometers.